Hydraulics & Water Calculators
20 free hydraulics & water calculators — standards-based, with formulas, worked examples, and no sign-up.
Water problems split into moving it, storing it, and getting rid of it — and this category spans all three at two very different scales. On the civil side, open-channel flow implements Manning's equation with hydraulic radius and Froude classification; rational-method runoff (Q = CiA, with Kirpich and FAA times of concentration) feeds the stormwater drainage and detention basin tools; culverts follow the FHWA HDS-5 inlet/outlet-control procedure; and gravity sewers are checked for full-flow capacity, partial-flow d/D, and self-cleansing velocity. Weir flow, hydraulic jump, and water hammer (the Joukowsky surge equation with wave celerity) cover the structures and transients.
On the building side, plumbing supply and drainage are sized by the fixture-unit method of the IPC and EN 12056, pumps get a system curve, duty point, and NPSH check, and rainwater harvesting follows the BS 8515 yield-and-tank-size balance. The evapotranspiration tool computes Hargreaves ET0 and crop coefficients for irrigation demand — the number the sprinkler zone calculator then divides by your measured bucket-test GPM.
A run of homeowner-scale tools rounds the category out, each anchored to the rule its trade actually uses: septic drain fields sized from bedroom count and percolation-rate application rates, water softeners in grains per week, French drains as gravel-fabric-pipe quantities with a 1% slope check, gutters and downspouts from pitch-adjusted roof area, and pool volume and chemical dosing straight from the pool industry's multipliers and the ppm-to-pounds chemistry relation.
All Hydraulics & Water Tools
Plumbing & Drainage Sizing
Pipe sizing via fixture unit method with flow rate, velocity, and self-cleansing velocity checks.
Pump Sizing Calculator
System curve generation, duty point selection, and NPSH verification with affinity law calculations.
Stormwater Drainage Calculator
Rational method pipe and channel sizing with IDF curves and runoff coefficient analysis.
Rainwater Harvesting Calculator
Roof catchment yield (V = A·R·C·eff) and tank sizing with a month-by-month water balance.
Detention Basin Calculator
Stormwater detention storage via the Modified Rational Method with critical-duration search.
Water Hammer Calculator
Joukowsky pressure surge, wave celerity and critical period 2L/a for rapid and gradual valve closure.
Open-Channel Flow Calculator
Manning's equation uniform flow for rectangular, trapezoidal, triangular, and circular channels.
Weir & Notch Flow Calculator
Discharge for rectangular, V-notch, and Cipolletti thin-plate weirs per ISO 1438.
Culvert Sizing Calculator
Inlet and outlet control capacity for circular and box culverts per FHWA HDS-5.
Hydraulic Jump Calculator
Hydraulic jump in a rectangular channel: upstream Froude number, sequent (conjugate) depth, downstream velocity and Froude number, specific-energy loss, percentage dissipated, jump length and the USBR jump classification.
Evapotranspiration (ET0) Calculator
Reference crop evapotranspiration by the Hargreaves equation (FAO-56 alternative): extraterrestrial radiation from latitude and day-of-year, crop ET via the crop coefficient Kc, and the net irrigation requirement with monthly totals.
Gravity Sewer Design Calculator
Circular gravity sewer design with Manning's equation: full-flow capacity and velocity, proportional depth d/D at the design flow, partial-flow velocity, capacity utilization and a self-cleansing velocity check.
Rational Method Peak Runoff Calculator
Peak stormwater runoff by the rational method Q = CiA with an area-weighted composite runoff coefficient, time of concentration by the Kirpich and FAA formulas, and SCS/NRCS Curve Number runoff depth and volume, with peak flow in m³/s, cfs and L/s.
Gutter & Downspout Sizing Calculator
K-style gutter sizing and downspout counts from representative SMACNA-style practice capacities: roof plan area adjusted by pitch factors from 1.0 (flat to 3/12) to 1.3 (12/12), normalized to the 1 in/hr capacity basis with the local 5-minute 10-year rainfall intensity, 5-in (2,500 ft²) and 6-in (3,840 ft²) K-style adequacy with utilization and a smallest-adequate recommendation, and per-type counts of 2x3, 3x4, and 4-in round downspouts from both drainage capacity and the 40-ft advisory spacing.
Septic Drain Field Size Calculator
Septic drain field (leach field) sizing from representative published state tables with the local health department always governing: design flow at 150 gpd per bedroom (Maryland COMAR 26.04.02.05 residential minimum; NY allows 110 gpd/bedroom with post-1994 fixtures), application rate from the NY Appendix 75-A Table 4B percolation bands transcribed cell-for-cell (1.20 gal/ft²/day at 1–5 min/in stepping down to 0.45 at 46–60 min/in), required trench-bottom area as flow ÷ rate, total absorption trench length at a 2- or 3-ft width split into equal laterals of at most 100 ft, minimum septic tank guidance from NY Table 3 (1,000 gal through 3 bedrooms, 1,250 at 4, 1,500 at 5), unsuitable-soil guidance toward mound and alternative systems for perc rates faster than 1 or slower than 60 min/in, and a required-area chart across every perc band at the chosen bedroom count.
Water Softener Sizing Calculator (Grains)
Softener grain-capacity sizing from representative plumbing-trade practice: household demand at 75 gal per person per day, hardness in grains per gallon or ppm converted at 17.1 ppm/gpg, dissolved clear-water iron compensated at +5 gpg per 1 ppm (published guides commonly cite 4–5; iron above ~0.3 ppm often wants dedicated treatment ahead of the softener), the weekly grain load on a one-regeneration-per-week basis matched to the smallest fitting standard capacity from 24,000 to 80,000 grains, a utilization chart across all six sizes against the 100% line, and twin-tank or frequent-regeneration guidance when even the largest single-tank size is too small — all framed as estimates to confirm against the manufacturer’s sizing chart.
French Drain Calculator (Gravel, Fabric & Pipe)
French drain material estimator from representative drainage-trade practice: trench volume from width (6/9/12-in presets or 6–24-in custom), depth, and run length with the standard 4-inch perforated pipe displacement subtracted so stone the pipe already fills is not bought, washed drainage gravel in cubic feet, cubic yards, and tons at 1.4 tons per cubic yard (link to gravel-aggregate for other materials), non-woven filter fabric from the burrito-wrap girth — twice the width plus twice the depth plus a 12-inch lap, covering walls, bottom, and a full top pass — counted in 3 × 100 ft rolls at 90% overlap-effective coverage, perforated pipe with a 5% fittings allowance rounded up to 10-ft sticks, a slope check against the 1% practice minimum (0.12 in/ft, with the 0.5% smooth-wall nuance in prose) or the required fall when no drop is entered, and a gravel-tons-by-width comparison chart.
Sprinkler Zone Calculator (GPM & Zones)
Irrigation zone calculator from representative trade practice: available flow measured with a bucket-test helper (gallons ÷ seconds × 60, with a use-this button) or typical meter-size presets (12/16/25 GPM for 5/8-, 3/4-, and 1-inch meters, clearly labeled typical — measure yours), a 90% design margin on the measured supply so a hot-afternoon pressure dip does not starve the farthest heads, sprinkler-head rows by type (label, GPM each from the nozzle chart, count) summed into total demand, the zone count as total demand divided by usable flow rounded up per the Hunter design-handbook method, an advisory equal demand split, a per-head-type maximum-heads-per-zone fit table (epsilon-guarded floor), and a demand-per-zone bar chart against the budget line.
Pool Volume Calculator (Gallons & Liters)
How many gallons a swimming pool holds for rectangular, round, oval, and kidney shapes using the pool industry's published multipliers (Pentair engineering data / PHTA guidance): L × W × average depth × 7.5 for rectangles, dia² × average depth × 5.9 for rounds, full-axis L × W × average depth × 5.9 for ovals, and 0.45 × (A+B) × L × average depth × 7.5 for kidneys, with average depth from the shallow/deep pair or a constant-depth toggle, liters at 3.78541 L/gal, water weight at 8.34 lb/gal, and an optional first-fill cost from the $/1,000 gal water rate on your utility bill.
Pool Chemical Calculator (Chlorine, Alkalinity, Hardness & CYA)
Chemical doses that RAISE free chlorine, total alkalinity, calcium hardness, or cyanuric acid from the pure-chemistry relation dose lb = gallons × Δppm × 8.34 ÷ 10⁶ ÷ strength factor: chlorine from cal-hypo 65/73%, dichlor 56%, or trichlor 90% in lb + oz, or 12.5% liquid chlorine in fl oz + gallons via the w/v trade-percent rule; sodium bicarbonate at the CaCO₃-equivalence factor 0.5957 (1.4 lb per 10 ppm per 10,000 gal); calcium chloride at 0.9019 × purity (94% anhydrous / 77% dihydrate); CYA at factor 1; with the dichlor/trichlor stabilizer side-effect reported per dose and pH lowering deliberately excluded (nonlinear acid demand — use the test-kit acid-demand table).
Frequently Asked Questions
Rational runoff, stormwater drainage, or detention basin — where do I start for site drainage?
They chain. Rational runoff gives you the peak flow Q from catchment area, runoff coefficient, and rainfall intensity. Stormwater drainage takes that Q and sizes the pipes and channels that convey it. The detention basin tool closes the loop when your jurisdiction caps release rates — it finds the storage volume needed to hold the difference over the critical storm duration.
Why does the sewer tool care about a minimum velocity, not just capacity?
Gravity sewers must self-cleanse: below roughly 2 ft/s (0.6 m/s) at design flow, solids settle and the line silts up. That is why the tool reports partial-flow velocity via the d/D ratio, not just full-pipe capacity — a generously oversized sewer laid flat can fail by being too slow, not too small. The same self-cleansing check appears in the building drainage tool for the same reason.
Can I size a pump straight from the pipe calculator's pressure drop?
Not quite — friction loss at one flow is a single point. A pump has to be matched against the whole system curve (static head plus friction that grows with flow squared), which is what the pump sizing tool builds. It then intersects your pump curve to find the true duty point and checks NPSH so the intake does not cavitate. Use pipe/duct sizing for the friction inputs, then hand off.